memfd-bind: more specific doc URL

Let's point to the relevant README directly in the systemd unit file,
as it is hard to find in the whole nine yards of the runc repo.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

contrib/cmd/memfd-bind/[email protected]

@@ -1,6 +1,6 @@
 [Unit]
 Description=Manage memfd-bind of %f
-Documentation=https://github.com/opencontainers/runc
+Documentation=https://github.com/opencontainers/runc/blob/main/contrib/cmd/memfd-bind/README.md
 
 [Service]
 Type=simple